Author Topic: 1987 K100LT rough idle and some rough running lower rpm (Read 13671 times)

Skids · « **on:** December 05, 2015, 10:30:06 AM »

I am just back to riding after a 40 year hiatus. I bought an OK budget K100LT that is cosmetically rough but seemed to run OK. Now I'm doing the basic maintenance to make sure it will last. I've ridden several hundred miles and glad to be back riding again.

It starts up fine, smooth idle at 1k rpm. Once it gets warm, it idles moving between 1000 and 1750 rpm. Also, when riding at lower rpms, like 3-4K or so, it seems to misfire occasionally, causing the bike to jerk a bit. When at higher speeds and rpms it seems much smoother.

Any suggestions about what may be going on with the idle. I replaced the spark plugs but the problem remains.

Thanks,

Skids

Scud · « **Reply #1 on:** December 05, 2015, 11:12:17 AM »

Have you replaced the little rubber caps on the throttle body synch tubes? If those are cracked, you can introduce extra air into the system. Extra air can cause low-rpm problems that go away at higher RPMs.

The Mighty Gryphon · « **Reply #2 on:** December 05, 2015, 11:19:25 AM »

There is a breather tube that conect to the airbox just behind the throttle position sensor. It's about 3/4" in diameter and makes a sharp bend where it enters the box. These things are notorious for cracking and introducing unmetered air into the engine. Check it before you do anything else.

Also, you might want to check the bushings above the throttle bodies for cracks and leaks as well. The rubber on these old bikes is nearing the end of it's useful life.

Elipten · « **Reply #3 on:** December 05, 2015, 11:41:50 AM »

All rubber components are suspect with the age of these bikes. The above two suggestions are a great start for easy and common issues.

Plug wires are old also. Take the coil cover off and the plate covering where the plugs enter the head. Run it at night in dark location and look for sparks. Then spray with water and look again

rbm · « **Reply #4 on:** December 05, 2015, 12:48:54 PM »

In addition to what Elipten said, pull the plug leads with the cover off to see if the spark plug nipples are on and intact. There have been cases reported here when poor electrical contact between the plug and wire occur because of missing those replaceable nipples.

BTW, "bushings" are the BMW name for the rubber bits that join the airbox to the throttle body assembly and for the rubber bits that join the throttle body assembly to the engine block. They live a harsh life inside the fairing and do crack with age, as mentioned.

One other item that might be causing your mid-range hiccup is a clogged fuel filter. If you don't know the service history of the bike, then replace this item as a matter of course. There are numerous substitutes available that can be obtained at your local aftermarket automotive supply shop.

Skids · « **Reply #5 on:** December 05, 2015, 04:22:53 PM »

Thank you all for the suggestions. I will run these down to see if they help. Actually I am just now noticing that my throttle body sync tubes, 2 of 3 have no caps and the one cap that is there is cracked a bit.

So perhaps that there is the culprit. I suppose I find those on line at a place like beemerboneyard or?

Thanks again!

rbm · « **Reply #6 on:** December 05, 2015, 04:27:13 PM »

Any auto supplier like Napa, Pep Boys or similar will have those caps in stock. Pretty standard off the shelf stuff.

Scud · « **Reply #7 on:** December 05, 2015, 04:27:21 PM »

Take the one cap you have to any auto parts store. $3 from now your idle will be improved. But don't stop there. Check all the rubber bits as others pointed out. You can get away with electrical tape over cracked rubber just to test whether it's leaking - but that's not a long-term solution.

Skids · « **Reply #8 on:** December 05, 2015, 06:37:34 PM »

Scud, Gryphon, Elipten and rbm,

Thanks again! I changed the oil and filter this evening, and then covered the sync tubes with electrical tape and took it for a ride. The problem is solved. So now the idle is steady, albeit seems a bit high. Is there some standard to have these bikes idle at?

As suggested I'll get the proper caps ASAP as well as the other rubber bits that are rotted.

One more thing if you have ideas - when I twist the throttle open, it doesn't immediately return when I loose it. It stays on and then will gradually reduce unless I twist it off. I'm pretty sure it should return immediately. I seem to remember reading not to lubricate the cable. Any suggestions on how to fix this?

And thanks again to you all,

Skids

Scud · « **Reply #9 on:** December 05, 2015, 07:06:53 PM »

Quote from: Skids on December 05, 2015, 06:37:34 PM

One more thing if you have ideas - when I twist the throttle open, it doesn't immediately return when I loose it. It stays on and then will gradually reduce unless I twist it off. I'm pretty sure it should return immediately. I seem to remember reading not to lubricate the cable. Any suggestions on how to fix this?

Great progress.

For your next question: Check the whole throttle and linkage for free play, corrosion, worn parts, etc. I'd start at those springs between the throttle bodies in your picture. One looks clean and the other not so much... Spray some cleaner/lube in there and work it around with a toothbrush. Maybe disconnect the throttle cable to see if the linkage snaps back freely. If it does, then your problem is probably in the cable or handlebar.

I'm pretty new to the K-bikes too (I've had mine for a only a month), but they're very logical and nicely laid out. Since you're just getting started with a bike that's "a bit rough" and are doing the fluids, I'll make another suggestion: I used RedLine Heavy Duty Shockproof Gear Oil in my transmission and it made a noticeable, positive difference. Much smoother shifts and easier to find neutral. I'm going to put some of that in my final drive too. My bike was cosmetically a bit rough too - gonna be chipping away at it over the next few months.

Laitch · « **Reply #10 on:** December 05, 2015, 07:28:19 PM »

No doubt about it, Skids; the throttle should snap back.

Lubricate the throttle cable gear on the handlebar. The following link will help you although a couple of images are missing. Follow the text and consult the pdf linked within the text. http://www.motobrick.com/index.php/topic,1797.0.html

Also, there might be a screw with a knurled head and a spring on its shaft located on the underside of the throttle tube. It applies friction to the throttle tube. Some riders use it as a cruise control. Back it off if it's making contact and slowing return of the throttle.

Scott_ · « **Reply #11 on:** December 05, 2015, 08:28:46 PM »

Additional to look for with the sticky throttle, check and make sure that the grip rubber isn't rubbing the switch housing.
I have read of some installing the rubber grip too far onto the tube and then it would rub and drag.

Skids · « **Reply #12 on:** December 05, 2015, 08:37:42 PM »

Thank you motobrickers for the throttle advice. I will get to work in the morning with your expertise in mind and let you know how it works out.

Cheers,

Skids

rbm · « **Reply #13 on:** December 05, 2015, 10:25:45 PM »

Also make sure that the throttle cam between TB2 and TB3 is not interfering with the lower TB bushing (hanging up on the screw clamp for example). Unlikely but possible.

Elipten · « **Reply #14 on:** December 05, 2015, 11:19:44 PM »

All good suggestions and valid. This is a great group of gents.

As suggested, I would check the knurled screw is not engaged on the throttle grip. If that is not the issue then disconnect cable to throttle bodies and move it by hand. If moves freely and snaps closed the move to cable and grip

I helped a young man earlier this year and his issue was junk grips

K1300S · « **Reply #15 on:** December 06, 2015, 07:40:46 AM »

Quote from: Skids on December 05, 2015, 10:30:06 AM

I am just back to riding after a 40 year hiatus. I bought an OK budget K100LT that is cosmetically rough but seemed to run OK. Now I'm doing the basic maintenance to make sure it will last. I've ridden several hundred miles and glad to be back riding again.

It starts up fine, smooth idle at 1k rpm. Once it gets warm, it idles moving between 1000 and 1750 rpm. Also, when riding at lower rpms, like 3-4K or so, it seems to misfire occasionally, causing the bike to jerk a bit. When at higher speeds and rpms it seems much smoother.

Any suggestions about what may be going on with the idle. I replaced the spark plugs but the problem remains.

Thanks,

Skids

suggest buying a shop manual for the bike asap. Clymers, Haynes, or just download the bmw manual from this forum. go through the major maint process outlined in first part of the books. doing that will get the bike to a known baseline you can troubleshoot from.

Quote from: The Mighty Gryphon on December 05, 2015, 11:19:25 AM

There is a breather tube that conect to the airbox just behind the throttle position sensor. It's about 3/4" in diameter and makes a sharp bend where it enters the box. These things are notorious for cracking and introducing unmetered air into the engine. Check it before you do anything else.

Also, you might want to check the bushings above the throttle bodies for cracks and leaks as well. The rubber on these old bikes is nearing the end of it's useful life.

the crankcase tube was a leading cause of my fast idle. cracked almost all the way around on both ends.

replaced tube, synched tb's, set choke, set idle, all good!

rbm · « **Reply #16 on:** December 06, 2015, 09:30:31 AM »

Quote from: Skids on December 05, 2015, 06:37:34 PM

So now the idle is steady, albeit seems a bit high. Is there some standard to have these bikes idle at?

Yes. 950 RPM +/- 50 RPM

Skids · « **Reply #17 on:** December 06, 2015, 05:13:03 PM »

Gents,

Thanks again for the advice. Got the sync tube caps replaced as well as the crankcase tube with new parts. This did the trick for the rough idle. It runs great now! Working on the throttle problem - cleaned up the springs, etc, on the throttle body but it still sticks so I will follow some of the other suggestions regarding corrosion, the cable, etc. And while it is now running smoothly, it is idling a bit fast so will need to adjust it as some point.

I rode it a bit over the weekend and the rear shock seems pretty weak, so attending to that is on my list now.

Thanks again,

Skids

Martin · « **Reply #18 on:** December 06, 2015, 06:29:17 PM »

I know you are not supposed to lube the cables, but I have had good success with a product called Tri Flow PTFE Teflon ( Used on bicycles ).It is the only thing I ever use on mine, and since I started using it probably 14 years ago I have not replaced a cable (100,000 Km).Could be just a coincidence.When I use it, I undo cable at the high end,you can then make a funnel out of plasticine or use one of those cable lube devices. I then flush out the cable until it runs clean.I do this approx once a year.
Regards Martin

Skids · « **Reply #19 on:** December 06, 2015, 06:40:41 PM »

Thanks Martin, and you other Motobrickers.

I just sorted my throttle issue. A prior owner had put a "Supervista" throttle lock over the twist grip. It seemed to work ok without any binding. However I removed it and found it had messed the grip up pretty badly. After removing the device, and trimming the grip a bit, the throttle now snaps back as it's supposed to.

Now I need some new grips. Any suggestions on good ones and from whence to order them.

Cheers,

Skids

rbm · « **Reply #20 on:** December 06, 2015, 08:29:15 PM »

Duck recommended Progrip 699 grips in the past. Most any will fit though.

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Author Topic: 1987 K100LT rough idle and some rough running lower rpm (Read 13671 times)